Hello all, I've been browsing the forums for maintenance and minor issues I've had since I bought my LS400 about two years ago. For some background it has 270k miles on it now and I bought it with 240k. Have kept up on the basics for the most part, fluids and all that.

Just the other day though I was driving and the car seemed okay until I stopped at a light. My check engine came on and was flashing, accompanied by the VSC light (as I've learned likes to come on for about anything except the VSC itself in my experience). I took it to Autozone and did a OBD scan using their equipment and had a handful of codes thrown at me.

I'll list them here for ease of reading:
​​​​​​P0171 System too lean Bank 1
P0300 Random misfire detected
P0305 Cylinder 5 misfire
P0306 Cylinder 6 misfire
P0171 System too lean bank 1 (this was listed twice on the scan so I figured I would list it twice as well
P0303 Cylinder 3 misfire.

Autozone's computer recommended solution is repairing a vacuum leak at the intake manifold but I don't know how much that is worth. Anyway the car is definitely feeling sluggish especially at low speeds, got awful mileage on the way home, and has a sound that definitely sounds like a vacuum leak when accelerating.

The thing that concerns me most is the misfires and the only thing I can think would be spark plugs for those, unless it is the intake and it's just affecting multiple cylinders? I've lost my job due to all the Corona virus stuff going on so I can't afford to throw a bunch of money at right now and it's my daily driver normally. I was thinking of getting spark plugs and checking the manifold while I'm in there but I wanted to post and see if anyone had any other recs.

 

Check all vacuum hoses and fittings.

Bank 1 upstream oxygen sensor?

Mass Airflow (MAF) sensor?

 

Do you think those in addition to the spark plugs or before even messing with them? Gonna be taking a look this evening when it's a little cooler out (I'm in Phoenix AZ)

 

Update from wrenching around on it today. There was a very loose fitting and soft PCV hose that I replaced, everything else hose wise looked good. Didn't get to looking at sensors yet. Still felt bad driving and all lights on still so I went ahead and replaced the 3 spark plugs from the misfiring cylinders, as well as a coil boot I ripped from cylinder 5 when I was removing it. After bolting everything back up it was night and day difference, much quieter idle besides a bit rough when cold and much smoother acceleration. Drove it around for awhile to wait on my CEL and sure enough it popped back up flashing, accompanied by the VSC. Took it to Autozone again and this time the scan showed only Cylinder 6 misfiring, though listed twice. I'm thinking maybe I should replace the boot on that one as well since it was only about $15 and double check the wiring from the cap while i'm in there. I very well could have just installed that one badly as well lol.

Any new thoughts or opinions? I'm pretty well confident it's nothing major or even very time consuming at this point, #6 was very easy to get to compared to the rest haha. Once I get this cleared i'm going to replace the remaining spark plugs as well.

 

I would perform a compression test on all of the cylinders. This will tell you if the engine is fundamentally good or not. Personally I wouldn't throw too much money at it until I did that.

 

More updates after a cool evening here finally. First the bad.

​​​​​​.. Got a power steering leak somewhere haha. The good being that after messing with trying to clean connectors and other stuff I finally got around to just pulling the ignition coil off #s 4 and 6 and swapped them. And the misfire followed it to #4. So I just ordered one online seeing as every parts store around me was trying to charge nearly $100 for a single coil. Tomorrow morning I'm going to look around for the power steering leak before it gets hot out and hopefully find it without too much trouble. Topped it off tonight and saw a big puddle pretty quickly. I plan to just throw some in and hopefully be able to see where it's from

 

The high pressure hose at the crimp is likely the leak area. Mine did that recently and after driving I'd find a 4" sized circle just inward of the front right wheel. With the car running I could see it visibly leaking a fair amount.

 

Fortunately the PS pressure hose from the pump is a pretty straightforward DIY fix. In my case I got the high pressure part from edelman on rockauto for about fifty bucks and installed it in an hour or so.

 

Good.

 

Coil showed up today and I drove around for awhile (after putting some ATF in my power steering) and the car is running like it was before all this. Going to do my remaining plugs and coils here soon before they start acting up but firstly I'll examine the power steering today after it cools off a bit. It looks like I'll have to jack the car and I'm not too keen on being under it on the pavement in 113 degree heat.

 

Wouldnt hurt to check the P/S valve that has vacuum hoses leading to the intake as well while you're under there. Those valves tend to leak as well not only outside but into the throttle body too. Did you clean the MAF by chance? after 20 years it's definitely dirty.

Cheers

 

I was going to suggest a loose coil plug connector, 6 out of 8 of mine have broken shells when I did my Denso TT spark plug replacement so i had to use 2 small zipties on each (two for extra insurance) to get them to plug in securely.

The coolant temp sensor would also be another suggestion for better performance/mpg as that dictates how the engine operates when cold//hot.

The power steering leak is likely the idle up valve that screws onto the pump, it has two vacuum lines coming off of it. About $60 for the part if its never been done before.

 

P/S valve? Did you mean idle control valve? I don't think it causes misfire.

 

No, I'm referring to the power steering valve. it has vacuum hoses that lead into the intake. if its gone bad, it will leak power steering fluid into your intake, which can cause missfire. Extreme case, but just an idea.